This invention relates to the field of optical signal processing, and in particular to the processing of packet-based optical signals and to the reading and updating of the routing information and the like contained in such packets, typically in headers of such packets.
The advantages of packet-based optical transmission systems are well known, and the SONET standard is widely accepted as a suitable physical layer for optical transmission systems. Indeed, SONET is one of the many physical layers defined for ATM, which is itself a cell-based (i.e. packet-based) switching and multiplexing technology.
The rapid increase in transmission rates achieved by optical transmission systems far exceeds the capability of electronic processing of signals. Consequently, the limitation to data transfer rates results principally from delays introduced by electrical switching elements. However, these electronic and opto-electronic elements are required for performing switching and routing functions, and the conversion of high-speed optical data to electrical signals for the switching and routing operations is recognised as causing speed reductions.
There have been proposals to provide all-optical networks in which switching and routing take place in the optical domain. Thus U.S. Pat. No. 5,541,756 proposes the use of a packet header with wavelength-coded data, in conjunction with a grating operating as a wavelength differentiator. Different wavelength signals within the optical header are deflected by the grating by different amounts to be incident upon photo-electric sensors positioned at different locations.
In a number of applications it is desirable to be able to change the contents of a header at a switch located at an intermediate position between where an individual packet is first created and its final destination. While effecting such a change in the electric domain is relatively easily accomplished, it is not so easy to accomplish this in the optical domain. A disadvantage of converting the contents of an optical packet (or just its header) into electrical domain, modifying it, and then using a laser/modulator to reconvert it back into the optical domain, involves a number of disadvantages. For instance, the process is not optically transparent, and so wavelength information is lost. Additionally the delay involved in conversion and reconversion is significant. Furthermore, the process requires the provision of more capacity in the router to process the signals passing through. If, on the other hand, one remains in the optical domain employing amplitude shift keying (ASK), which is the preferred format for optical data transmission, then, while it is easy to convert a high level ASK bit to a low level one merely by gating, conversion in the opposite direction (low level to high level) requires the provision of a facility for injecting light into the system at the location of the conversion.
An object of the present invention is to provide a format of optical transmission network in which data packets, having a data-payload part and a non-payload that includes data packet routing information, can have their non-payload parts rewritten without having to have recourse to the use of an optical source at the location of such rewriting.
According to the present invention, there is provided an optical transmission network in which data packets, having a data-payload part and a non-payload that includes data packet routing information, are routed by an optical switch in different paths according to the routing information contained in the non-payload part of their respective data packets, and wherein the data-payload part of a data packet is transmitted as amplitude shift keying of an optical carrier while the non-payload part is transmitted as phase shift keying of the optical carrier.
Generally, but not necessarily always, the non-payload parts of such data packets take the form of data packet headers. There is thus no intrinsic reason why the non-payload parts of such data packets should not take the form of data packet footers.